This invention relates to electroluminescent devices, especially ones that have a conjugated polymer as a light-emitting layer.
One type of electroluminescent device is described in PCT/WO90/13148, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(p-phenylenevinylene)xe2x80x94xe2x80x9cPPVxe2x80x9d) sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. It is believed that the electrons and holes excite the polymer film, emitting photons. These devices are useful as flat panel displays, in which case one of the electrodes is transparentxe2x80x94for example being made of indium-tin oxide (xe2x80x9cITOxe2x80x9d).
One refinement of the device is that to allow the device to display information one of the electrodes can be patterned so that it only injects charge carriers over part of its area. The electrode can be patterned into discrete areas which can be controlled independently and each inject charge carriers into a respective discrete pixel of the polymer film.
Another refinement is that it has been found that the turn-on voltage, the brightness of the device at low voltage, the efficiency, the lifetime and the stability of the device can sometimes be improved by adding a layer of conductive polymer between the light-emitting film and one of the electrodes. In order to achieve these benefits these conductive polymer layers typically have a sheet resistance less than 106 Ohms/square, the conductivity being controllable by doping of the polymer layer. One example of a suitable conductive polymer is polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDT-PSSxe2x80x9d)xe2x80x94see EP 0,686,662. For reasons which are discussed in more detail below, it is, however, difficult to produce good devices with an intermediate conductive polymer layer.
There have been some devices which could be regarded as a hybrid of these refinements. For instance, U.S. Pat. No. 5,399,936 discloses a device in which the patterned electrode is covered by a hole-transporting/injecting organic layer of diphenyl-N-Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (xe2x80x9cTPDxe2x80x9d). However, the TPD is a small molecule, not a polymer, and it is undoped. WO 96/08047 discloses a device in which a conductive polymer layer is patterned to provide insulative regions and conductive regions to pixellate the display.
According to the present invention there is provided an electroluminescent device comprising: a first charge-carrier injecting layer for injecting positive charge carriers and a second charge-carrier injecting layer for injecting negative charge carriers, at least one of the charge-carrier injecting layers being patterned so as to comprise spaced-apart charge-injecting regions; an organic light-emitting layer located between the first and second charge-carrier injecting layers; and an unpatterned conductive polymer layer located between the organic light-emitting layer and the patterned charge-carrier injecting layer, the resistivity of the conductive polymer layer being sufficiently low to allow charge carriers to flow through it from the charge-injecting regions to generate light in the organic light-emitting layer but sufficiently high to resist lateral spreading of charge carriers beyond the charge-injecting regions.
The patterned charge injecting layer could be an anode or a cathode. The other charge injecting layer (of opposite polarity) may be patterned or unpatterned. The width of the charge injecting regions is suitably less than 1 mm, 300 xcexcm or 100 xcexcm, preferably less than 50 xcexcm and most preferably less than or around 20 xcexcm. The spacing between the charge injecting regions is suitably less than 1 mm, 300 xcexcm or 100 xcexcm, preferably less than 50 xcexcm and most preferably less than or around 20 xcexcm.
The cathode is preferably a sputtered cathode. The cathode is preferably a low work function cathode. The cathode may be patterned by dry etching.
The light-emitting layer preferably is or comprises a semiconductive conjugated polymer such as PPV. The light-emitting layer suitably is or comprises PPV, poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), a PPV-derivative (e.g. a di-alkoxy derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers. It could be deposited by spin-coating, dip-coating, blade-coating, meniscus-coating, self-assembly etc. The constituent of the light-emitting layer and/or its precursor is suitably water-based: examples are precursor-based PPVs. The thickness of the light-emitting layer is preferably in the range from 20 to 200 nm and most preferably around 100 nm.
The term xe2x80x9cconjugatedxe2x80x9d indicates a polymer for which the main chain is either fully conjugated, possessing extended pi molecular orbitals along the length of the chain, or is substantially conjugated, but with interruptions to conjugation at various positions, either random or regular, along the main chain. It includes within its scope homopolymers and copolymers.
The conductive polymer layer preferably is or comprises polyethylene dioxythiophene (xe2x80x9cPEDTxe2x80x9d), polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDT-PSSxe2x80x9d), doped polyaniline, a doped alkylthiophene and/or a doped polypyrrole. The thickness of the layer is suitably less than 200 nm, preferably less than 100 nm and most preferably less than or around 50 nm. The sheet resistance of the layer is suitably greater than 106 or 107 Ohms/square, preferably greater than 108 Ohms/square and most preferably greater than or around 1010 Ohms/square.
The conductivity of the conductive polymer layer is preferably adjustable (e.g. by means of additive components) over a relatively wide range. The conductive polymer is preferably doped with one or more components that influences its conductivity (e.g. polystyrene sulphonic acid (xe2x80x9cPSSxe2x80x9d) or epoxy-silane). The conductive polymer is suitable polyaniline, polypyrrole or a poly-thiophene.
The conductive polymer is preferably adapted to reduce its solubility and/or its interaction with an adjacent layer, especially a polymer layer. This layer is preferably the subsequent layer and most preferably the light emitting layer. To achieve this the conductive polymer preferably contains an additional component. This component is suitably a cross-linking agent (e.g. epoxy-silane). This component preferably does not significantly impair the performance of the device, especially by increasing the drive voltage significantly, reducing the emission uniformity significantly or reducing the efficiency significantly. This component is especially advantageous in a device in which the conductive polymer and the light-emitting layer have the same or similar solvents. The amount of this component in the conductive polymer layer is suitably greater than 50 wt %, preferably between 50 wt % and 200 wt % and most preferably around 100 wt % relative to the conductive polymer in the layer.
Where the conductive polymer is PEDT-PSS the dilution of the PEDT-PSS with the PSS is preferably greater than 20% and most preferably greater than 50%. Any of these proportions may be used together with any of the proportions of epoxy-silane disclosed above.
The resistance of the conductive polymer layer is suitably such as to require an increase in the drive voltage of the device (compared to when the layer is absent) of 0.5V or less (preferably 0.2V or less) for equivalent device performance.